1. Field of the Invention
The present invention relates to a process for producing a printed-circuit board from a metal-coated resin board by the aid of electroless plating.
2. Description of the Prior Art
According to the generally accepted conventional practice, flexible printed boards and tape automated bonding (TAB) boards are produced by forming printed conductors on a copper-coated resin film by etching. The copper coated resin film is composed of an insulating film of polyimide resin or the like and a copper foil bonded to it with an adhesive of epoxy resin or the like. The etching of the copper coating is accomplished by photoetching or by chemical etching which involves the steps of forming a protective coating on the copper coating by screen printing and removing those parts of copper coating not covered by the protective coating. A disadvantage of the conventional process is that the adhesive is usually inferior in chemical resistance to polyimide. It absorbs impurities such as chlorine ions while the printed conductors are being formed and it retains them even after washing. This often leads to the incomplete insulation of printed conductors, especially in the case of dense printed conductors.
To overcome this disadvantage, there was proposed a process for forming a copper coating directly on the resin board without using any adhesive, as disclosed in Japanese Patent Publication No. 44597/1981. According to this process, printed conductors are formed by the steps of forming a first thin copper layer by electroless plating, applying a photoresist to the copper layer, exposing the photoresist through a desired pattern, developing the photoresist, forming a second copper layer on the exposed part of the first copper layer by electrolytic plating until a desired thickness is reached, removing the photoresist, and etching the entire copper coating until the first copper layer is removed.
The above-mentioned process, however, still has a drawback resulting from the use of a chemical for catalyst activation. (Electroless plating needs a pretreatment with palladium or the like to activate the catalyst.) Part of the chemical used for catalyst activation remains unremoved in the subsequent steps, adversely affecting the insulation of printed conductors.